Mortise & Tenon
Simple and strong, the mortise and tenon joint has been used for thousands of years by woodworkers around the world to join pieces of wood, usually when the pieces are at an angle close to 90°. Although there are many variations on the theme, the basic idea is that the end of one of the members is inserted into a hole cut in the other member. The end of the first member is called the tenon, and it is usually narrowed with respect to the rest of the piece. The hole in the second member is called the mortise. The joint may be glued, pinned, or wedged to lock it in place.
Mortise and Tenon Variations
- Open mortise – a mortise that has only three sides. (See Bridle joint).
- Stub mortise – a shallow mortise, depth depends on the size of the timber; also a mortise that does not go through the workpiece (as opposed to a “through mortise”).
- Stub tenon – a short tenon; depth depends on the size of the timber; also a tenon that is shorter than the width of the mortised piece so the tenon does not show (as opposed to a “through tenon”).
- Tusk tenon – a kind of mortise and tenon joint that uses a wedge-shaped key to hold the joint together.
- Through tenon – a tenon that passes entirely through the piece of wood it is inserted into, being clearly visible on the back side.
- Teasel tenon – a term used for the tenon on top of a jowled or gunstock post, which is typically received by the mortise in the underside of a tie beam. A common element of the English tying joint.
- Top tenon – the tenon that occurs on top of a post.
- Feather tenon – a round-shouldered machined fillet or feather which is glued into a machine (router) made slot or mortise on each side of the joint.
Generally the size of the mortise and tenon is related to the thickness of the timbers. It is considered good practice to proportion the tenon as 1/3rd the thickness of the rail, or as close to this as is practical. The haunch, the cut away part of a sash corner joint that prevents the tenon coming loose, is one third the length of the tenon and one sixth of the width of the tenon in its depth. The remaining two-thirds of the rail, the tenon shoulders help to counteract lateral forces that might tweak the tenon from the mortise, contributing to its strength. These also serve to hide imperfections in the opening of the mortise.